This invention relates to pushbutton electric switches. More particularly, the invention relates to pushbutton electric switches wherein the pushbutton/plunger are manually depressed to bring butt type contacts into engagement. This type of contact action provides little or no wiping action desirable to keep the contact surface clean and free of foreign matter, e.g. carbon, that may impair normal current flow through the switch.
Pushbutton switches of the aforementioned type are relatively uncomplicated, consisting of a minimal number of parts and relying on predetermined depression of the plunger to achieve desired contact pressure. The plunger operates against a return spring and carries the movable contacts into abutting engagement with stationary contacts upon depression. Continued depression of the plunger subsequent to the abutting engagement effects compression of a contact pressure spring. The amount of compression of the pressure spring and the pressure provided thereby are each a function of plunger travel. Through proper selection of springs and plunger stroke, significant force can be applied to the contacts. However, foreign particles from switch arcing or ingress, accumulating on the contact surface, diminish electrical conduction properties between the contacts regardless of the pressure applied to the contacts. It is therefore advantageous and desirable to impart wiping action between the contacts during switch operation to scrape off any impurities collecting on the contact surface, thereby to provide a clean surface for maximum conduction. Inasmuch as the pushbutton switch is of simple straightforward construction, any mechanism for providing contact wipe should have similar construction.
Examples of obtaining contact wipe in pushbutton or plunger type switches are known in the art. For instance, U.S. Pat. 4,650,935 issued Mar. 17, 1987 to Ootsuka et al discloses a bridging contact carried by the linearly movable carrier of an electromagnetic relay wherein the carrier may be provided with oblique grooves and the bridging contact provided with tabs slidably received in the grooves for effecting lateral displacement of the contact as the carrier is moved relative to the bridging contact following abutting engagement with stationary contacts. The shallow angle of the grooves permissible within the carrier provides little lateral displacement of the bridging contact, particularly when the travel of the contact carrier after engagement of the contacts is shown to be approximately only one-third the length of the oblique grooves. As mentioned, this contact structure is described in conjunction with an electromagnetic relay wherein multiple bridging contacts are provided in the carrier. The contact force applied by springs bearing on the bridging contacts is necessarily limited because the cumulative force of these springs upon compression counteracts the closing force of the electromagnet and must necessarily not exceed the pull-in force thereof.
Another pushbutton switch is shown in U.S. Pat. No. 2,605,375 issued July 29, 1952 to G. S. Ellithorpe. This switch has a movable contact having outwardly projecting wings which engage angled surfaces of stationary contacts to wipe along those surfaces as the plunger is biased to an extended position. Contact pressure in this embodiment is applied by the return spring, such pressure reducing as a function of the extended length of the spring. Therefore, in the extended position of the switch, the pressure applied to the movable contact by the spring is at its minimum value. Second angled surfaces are provided for engagement by the movable contact on the following depression stroke to rotate the movable contact into preferred alignment with the first-mentioned angled surfaces for wiping action on the extending stroke as previously described. In this device, wiping action between the movable and stationary contacts is a direct function of axial travel of the pushbutton/plunger, and rotary movement of the movable contact is solely a by-product of such axial movement of the movable contact along the angled surface of the stationary contact.
U.S. Pat. No. 3,539,749 issued Nov. 10, 1970 to A. M. Macpheat discloses a bridging contact comprising dual contact elements having angular slots respectively reversely oriented. A pin carried by the pushbutton is forced linearly downward within the angular slots, thereby camming the contact elements in opposite lateral directions after the elements engage stationary contacts. A spring housed within a cavity of the pushbutton bears upon the contact elements to be compressed as a function of button travel, thereby applying pressure on the contacts.
U.S. Pat. No. 2,422,097 issued June 10, 1947 to K. H. Hansen discloses a pushbutton switch which shows pins radially projecting into angularly disposed slots in tubular members. However, this structure does not provide rotation of the contacts for contact wipe. The pushbutton is depressible axially and is non-rotatable to force a radially extending pin against an angular slot in a rotatable, but non-axially translatable intermediate member whose rotation then effects axial translation of a non-rotatable contact carrier to move the carrier axially into and out of engagement with stationary contacts without rotation U.S. Pat. No. 2,083,118 issued June 8, 1937 to B. Goldstone shows a pushbutton having a tongue movable in an oblique slot to alternately align the tongue with a non-conductive or a conductive surface of an electrical contact member according to the axial position of the pushbutton. U.S. Pat. No. 2,155,765 issued Apr. 25, 1939 to G. J. Meuer discloses a movable contact pin which is guided for vertical movement within slots in an insulating housing. Vertical movement is effected by translational movement of the pin within a slot in the button actuator which is pivotally depressed. The contact actuation occurring in this switch is strictly abutting action and provides no contact wipe.